In a heavy large skillet over medium-high heat, melt 2 tablespoons of butter and saute mushrooms until tender and beginning to brown, 3 to 4 minutes. Transfer to a bowl and set aside.
Bring chicken stock to a boil in a medium saucepan and keep it on a low simmer.
In a medium saucepan, heat oil and cook onions until softened and translucent, 8 to 10 minutes. Add the rice and stir with a wooden spoon until toasted and opaque, 3 to 5 minutes.
Add porcini (with the soaking liquid) and white wine, and cook, stirring constantly, until the liquid is absorbed.
Add hot stock to the rice, about ½ cup at a time, stirring often and making sure it is nearly absorbed before adding more.
Just before adding the last ladleful of stock, add the cooked mushrooms. The risotto is done when the rice is al dente and creamy, about 20 minutes. Stir in the butter and cheese until well mixed. Season with salt and pepper to taste and serve hot with extra cheese if desired.